It is well known in the superabrasive industry that composite polycrystalline superabrasive masses or compacts composed of diamond or cubic boron nitride (“CBN”) crystals are widely used in industry as cutting elements in cutting or dressing tools, wire drawing dies and drill bits.
Most HPHT processed superabrasive cutting materials, such as PCD and PCBN compact products, are prepared by being sintered onto a cobalt cemented WC substrate. This WC substrate provides a strong support layer due to its unique beneficial effect on PCD and PCBN thereby providing a blank comprising a PCD or PCBN layer firmly cemented at its interface to a WC-substrate layer. Such blanks can then be processed into forms or shapes suitable for their intended purposes as various superabrasive tools.
This commercially available cobalt (10-25 wt %) cemented tungsten carbide plays a vital role as a substrate layer for the sintering of PCD or PCBN abrasive layers and for promoting a primary diamond-diamond bonding microstructure in a PCD compact or a CBN-CBN bonding microstructure in a PCBN compact under HPHT conditions such as exemplified in Wentorf, U.S. Pat. No. 3,745,623. For example, cobalt infiltrated from a WC-substrate contributes as a binder for creating strong chemical diamond-diamond bonds having a desirable microstructure of PCD which is important to obtain optimum engineering properties in the formation of superabrasive cutting tools. It is also generally known that cobalt from a tungsten carbide substrate layer plays an important binder assisting role for direct CBN-CBN bonding in the formation of PCBN superabrasives on a substrate layer. Such may also apply as well for partly or indirectly binder-CBN bonding to a substrate layer, depending on the grades of PCBN products.
However, there may unfortunately be adverse effects to one or more of the engineering properties of PCD and PCBN abrasive layers bonded to a cemented WC-substrate layer in more detailed engineering structural perspectives.
One adverse effect to PCD or PCBN abrasives bonded to cemented WC-substrates is related to the residual stress generated at the PCD/WC or PCBN/WC interface during the HPHT PCD/PCBN sintering process which will directly influence the overall internal bulk stress inside the PCD or PCBN layer. Simply, the generation of residual stress at the interface of PCD or PCBN layer and the WC-substrate layer results from the strong HPHT joining of two different materials of different thermal properties even under the very high pressures and temperatures of the PCD/PCBN manufacturing process. Differences in thermal expansion and contraction in between the superabrasive (PCD or PCBN) and the tungsten carbide at the interface under stringent pressure and temperature, such as 50,000 atms and 1400° C. results in the formation of considerable amounts of residual stress in the PCD/WC or PCBN/WC interface. Furthermore, such differences directly influence the overall internal bulk stress on the PCD or PCBN layer.
Also, this undesirable overloaded internal stress on the PCD/PCBN layer is highly negatively influential to the additional tensile stress of PCD or PCBN during the follow-up post-pressing operations on PCD or PCBN for the final cutting tool manufacturing operations. For example in laser cutting into the PCD/PCBN segments, brazing as-cut PCD/PCBN segment into the tool body, and brazed PCD/PCBN tool shaping steps, such internal stress may induce cracks or fractures in either PCD/PCBN layer or at its interface with the WC substrate.
Middlemiss, U.S. Pat. No. 7,377,341 teaches a compact comprising an ultrahard abrasive joined to a substrate through an intermediate layer which allegedly provides improved matching of thermal expansion characteristics between the ultrahard material and substrate. Such intermediate layer can be provided as a preformed layer, such as a foil, or a green-state part which is joined to the ultrahard abrasive layer and substrate through HPHT processing.
Cho, U.S. Pat. No. 6,068,913 teaches the use of a corrugated intermediate superabrasive layer of PCD of high cobalt content which interconnects with the corrugated region of a corrugated tungsten carbide substrate layer for the purpose of forming a stress buffer area in between tungsten carbide substrate and the primary PCD abrasive layer.
Cho, U.S. Pat. No. 7,585,342 teaches a metal diffusion bridge matrix embedded in a superabrasive PCD or PCBN layer for bolstering bulk toughness and increasing wear resistance in the working superabrasive PCD or PCBN layer. Also taught is that such matrix, during HPHT processing, may form a metallic layer between the superabrasive and substrate layers which can serve to minimize inherent residual stress caused by the bonding of the dissimilar superabrasive and substrate materials.
While the above patents tend to offer some relief or solution to the residual stress problem inherent in the bonding of dissimilar materials of the superabrasive and substrate, there still remains a need for additional improvement.